Authors

Postdoctoral Research Fellow, Adelaide Institute for Sleep Health, Flinders University

Disclosure statement

Leon Lack is a shareholder in Re-timer, a company that makes light therapy devices. He receives funding from the ARC, NHMRC and Flinders University grants.

Gorica Micic receives funding from the Australian Research Council grant (Discovery Project DP120101401).

Nicole Lovato does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

The South Australian government is taking public submissions on whether the state should permanently change its time zone. The move came after Premier Jay Weatherill last month said being 30 minutes behind eastern time made South Australia a “joke anomaly”.

Aligning the state to Australian Eastern Standard Time is the main contender of several proposed changes. In all the discussion of the pros and cons for doing so, one important issue is yet to be considered: the effect of delaying our circadian rhythms (our internal body clock) and therefore reducing the amount of sleep we get.

It may have a small impact individually but it adds up to a substantial effect statewide. And our population tends to be sleep-deprived already.

Attention lapses

Extensive experimental research has shown that cumulative sleep loss produces feelings of tiredness that may subtly reduce motivation and enthusiasm for work, education and innovation in all walks of life. But these effects are eclipsed by the negative impact on thinking ability.

Thinking tasks that require continuous attention over time suffer the most. In real-life situations, this includes driving. Even mild sleep loss (around one hour) over many days results in an increasing number of lapses of attention. If this occurs while driving a vehicle, the result can be deadly.

If accidents involving injury and death occur as a result of changing the clocks in South Australia, this would represent a catastrophic cost to involved individuals. This cost would also be felt significantly statewide via the health-care system, insurance payouts and premiums, rehabilitation costs and so on.

How light affects our body clock

So, why would changing the clocks by half an hour result in widespread sleep loss?

It has to do with the timing of light exposure, which we know affects the timing of our underlying body clock. If we were to shift our clocks half an hour later, this would also delay the timing of our exposure to light by the same amount. Delaying our exposure to light will make it more difficult to get to sleep early enough in the evening to get sufficient sleep by the time most of us have to wake up in the morning.

Most of us schedule our morning wake-up time so we have just enough time to get ready and arrive at work or school, or help other members of our family to do so. After work, we have several hours of recreational time well into the night hours. Then we head to bed close to the middle of the dark night time and sleep in the second half of the night.

Let’s think about why our temporal life is arranged in such a peculiar way: why not sleep at the beginning of the night and have several hours before needing to start work?

The answer lies in how our body-clock timing is affected by light and how the vast majority of us have internal body clocks that tick over a bit slower than 24 hours.

Our body clock has a strong effect on when we feel sleepy or alert. The period before bed (let’s say 6pm to 10pm) is an alert zone of the body clock for most people. Just before awakening (4am to 7am, for instance) is the sleepiest zone of our body clock.

But our body clock is a little slower than the earth’s rotation. Carefully controlled experiments have shown that the time taken for our body clock to complete one full cycle is, on average, 24 hours and 15 minutes for the normal population.

So, our whole circadian system, including the alert and sleepy zones, tends to delay by about 15 minutes each day. In evening-type people, and especially in those suffering from delayed sleep phase disorder, their delay each day is even greater: up to an hour more.

What does this all mean for South Australia?

The tendency for our body clock to delay can be restrained and kept in check by visual light exposure. Exposure to bright light after waking is usually sufficient to stop our body from drifting any later.

But if South Australia adopts Australian Eastern Standard Time, it will result in the sun rising half an hour later than it currently does throughout the year. This reduction of morning light, like sleeping in late on the weekend, allows our body clocks to drift a bit later by clock time.

This delay of body clock will also postpone the time we feel sleepy at night and result in later bed times. Because wake-up times will remain the same clock time, the net effect will be less sleep. Sleep debts will accumulate, leading to the impairments we described earlier.

Daylight saving time during summer will exacerbate this effect by further delaying morning light and producing an additional body-clock-delaying effect of later evening light exposure.

However, South Australia has also been considering changing the clock in the other direction by 30 minutes. This would open the gap with most eastern states to an hour difference rather than 30 minutes but it would align SA with some Asian countries and Queensland in the summer.

This change would provide more morning light and should result in earlier bed times and more sleep for the population: a remedy for an already sleep-deprived population.